Thanks for very useful comments. Yes, vacuum region is indeed too low in this example (whoops, seems like I've attached an old input file). I also ran a calculation with 10 A of vacuum without any improvement. Open shell is unnecessary as it quickly converges to a closed shell solution. I've just wasted some CPU time with that.
Ectuwfc, ecutrhc are large enough (converged). I believe K points should also not be the source of a problem. As for DFT-D2, I admit I am a bit in the dark here. How 'wrong' is it? Or to put differently, is it more wrong that not using dispersion correction at all? How about vdw-DF, how do I turn it on and does it perform better? Where can I find more important about that? Also, if I change/switch off the dispersion correction, then I will have to re-optimize all already converged structures (metal, isolated molecules, adsorbed intermediates), right? Looks rather tedious Matej -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Ari P Seitsonen Sent: Tuesday, November 24, 2015 11:22 AM To: PWSCF Forum <[email protected]> Subject: Re: [Pw_forum] bfgs optimization not going anywhere Dear Matej Hus, Adding to the previous comments (I think that it is ok to use the k points, even though you might gain some CPU time by first indeed using Gamma-only and then increasing, as your cell is quite large), have you checked the length of your vacuum/lattice vector along the surface normal? If I see correctly, you have only about six Ångströms, which is already very (too) little, plus the DFT-D2 (Grimme-D2) adds quite strong an interaction through the vacuum at such distances I believe, plus the dipole correction... Greetings from Peaceful Montrouge/France, apsi PS My personal opinion of the vdW-DF* is, yes, they might be better, but first check the literature about a "suitable empirical choice of the particular mixture" of the functional, as some of the choices give quite large lattice constants already (4-5 % larger than experimental, which, of course, is not necessarily the target, rather a good adsorption geometry and energy). And often the best agreement is found by tuning the exchange functional, which naturally should have not nothing to do with the vdW/London dispersion, which is pure correlation... So much about "ab initio". ;) The good news of the day is that it seems that seldomly things seem to go completely wrong, no matter which choice of the vdW-DF* you take, but still, I would study a bit the literature first. If any one has a reference for a good review on the vdW-DF* et co applied to surfaces and adsorption, I would also be very thankful! :) -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*= - Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/ Ecole Normale Supérieure (ENS), Département de Chimie, Paris Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935 On Tue, 24 Nov 2015, Matej Huš wrote: > > Dear PW community > > I have a problem when trying to find optimized geometry of an adsorbed > intermediate in "benzene" hydrogenation on Ru surface (see input below). > > I believe I am fairly close to optimised structure since I'm only > adding one hydrogen atom to an already optimised reactant and only > slight displacing the adjacent atoms. > > > > The problem is that BFGS doesn't seem to go anywhere. Energy only > fluctuates, which to my understanding is a sign of being close to a minimum. > However, forces remain large. > > > > I've switched to the damp algorithm and the problem persists. I've > searched through the forum and the usual approaches of tackling this > issue do not seem to work. > > > > Any ideas? Thank you! > > Matej Hus > > Laboratory of Catalysis and Chemcial Reaction Engineering > > National Institute of Chemistry > > Ljubljana, Slovenia > _______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
